1. Field of the Invention
The present invention relates to the storage of digital data using an optical medium. More specifically, the present invention relates to a method and material for utilizing dispersed nanoparticles, linear electron transfer or nonlinear two-photon absorption to initiate second stage polymerization in volumetric optical data storage and, thus, store data by changing local reflectivity of a format hologram.
2. Background
Optical data storage technology has tended to follow two complementary lines of development. In one approach, data is encoded as minute variations in the surface of a recording medium, such as a compact disc, or CD. The data are readable using optical means (usually a laser), similar to the way in which data recorded in a magnetic medium are readable with a magnetically-sensitive head, or data recorded in a vinyl medium are readable with a needle. Unlike vinyl recording, however, in optical storage the data are usually stored digitally. For read-only compact discs, data are stored as microscopic pits on the surface of a substrate. In addition, recordable or re-writable bit-based optical systems are readily available. Examples include magneto-optic systems, in which the orientation of a magnetic domain changes the direction of rotation of the polarization of a reflected, focussed light beam; phase-change systems, in which a medium can be locally crystalline or polycrystalline, each of which states have a variance in reflectivity; and, dye-polymer systems, in which the reflectivity of a medium is changed by the high-power illumination.
Each bit of data has a specific physical location in the storage medium. The storage density of optical media is limited by physical constraints on the minimum size of a recording spot. Another basic limitation of conventional optical storage is that data are usually stored on the surface of the medium only. Recording throughout the volume of a storage medium would provide an opportunity to increase capacity.
Multi-layer storage is also possible, but usually requires the manufacture of special, heterogeneous, layered recording media, whose complexity increases quickly with the number of layers needed. Most commercially-available multi-layer optical storage media offer no more than two data layers, and come in a pre-recorded format.
An alternative approach to traditional optical storage is based on holographic techniques. In conventional volume holographic recording, laser light from two beams, a reference beam and a signal beam containing encoded data, meet within the volume of a photosensitive holographic medium. The interference pattern from the superposition of the two beams results in a change or modulation of the refractive index of the holographic medium. This modulation within the medium serves to record both the intensity and phase information from the signal. The recorded intensity and phase data are then retrieved by exposing the storage medium exclusively to the reference beam. The reference beam interacts with the stored holographic data and generates a reconstructed signal beam which is proportional to the initial signal beam used to store the holographic image. For information on conventional volume holographic storage, see, for example, U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669.
Typically, volume holographic storage is accomplished by having data written on the holographic medium in parallel, on 2-dimensional arrays or xe2x80x9cpagesxe2x80x9d containing 1xc3x97106 or more bits. Each bit is generally stored as information extending over a large volume of the holographic storage medium, therefore, it is of no consequence to speak in terms of the spatial xe2x80x9clocationxe2x80x9d of a single bit. Multiple pages can then be stored within the volume by angular, wavelength, phase-code or related multiplexing techniques.
Unfortunately, conventional volume holographic storage techniques generally require complex, specialized components such as amplitude and/or phase spatial light modulators. Ensuring that the reference and signal beams are mutually coherent over the entire volume of the recording medium generally requires a light source with a relatively high coherence length, as well as a relatively stable mechanical system. These requirements have, in part, hindered the development of inexpensive, stable, and robust holographic recording devices and media capable of convenient operation in a typical user environment.
In order for volumetric optical data storage to mature into a viable data storage option the process must be developed so that the operation is relatively simple, inexpensive and robust. Foremost in this development is accomplishing multi-depth bit-wise optical data storage and/or retrieval. As data recording proceeds to a greater number of depths within the storage medium it becomes increasingly more critical to isolate the recorded bit within a specific area within the medium. In multi-depth storage and/or retrieval, it is also important to write data at a given depth without affecting data at other depths. Further, for multi-depth bit-wise optical data storage and/or retrieval, it is important to have separate write and read conditions, so that readout does not negatively affect recorded data.
Briefly, and in general terms, the present invention comprises an improved optical data storage medium including a photopolymer for recording a hologram in a first stage polymerization under a first condition, and for recording data as localized alterations in the hologram at discrete, selected data storage locations within the storage medium in a second stage polymerization under a second condition. The photopolymer may comprise a first, hologram recording polymerization initiator, and a second, data recording polymerization initiator. In one preferred embodiment the hologram recording polymerization initiator generally comprises a linear absorbing sensitizer dye specific to a hologram writing wavelength or wavelengths, together with the first polymerization initiator, which may comprise a photoacid generator. In a first embodiment, the data writing polymerization initiator comprises nanoparticles dispersed throughout the photopolymer matrix. Light absorbed by the nanoparticles is converted to heat which initiates the chemistry required to write data as localized alterations to the format hologram. In a second embodiment the invention, the second stage polymerization initiator comprises a linear absorbing sensitizer dye, specific to a wavelength of the data writing or storage beam, which is homogeneously dissolved or dispersed throughout the photopolymer. In yet another embodiment, the data writing polymerization initiator exhibits a two-photon absorption mechanism, specific to a wavelength of the data writing or storage beam, which is homogeneously dissolved or dispersed throughout the photopolymer.
The invention further comprises a method for recording a format hologram and for recording data in an optical data storage medium. The method comprises the recording a format hologram in a photopolymer medium, by polymerizing monomer using a first, hologram recording polymerization initiator, and writing data by polymerizing monomer using a second, data writing polymerization initiator. The hologram recording polymerization initiator and data writing polymerization initiator are part of the photopolymer medium. The step of recording data may comprise one of three alternative polymerization initiating embodiments. In the embodiments described herein, given by way of example and not necessarily of limitation, each of the data recording methods rely on local polymerization changing the amplitude of the refractive index modulation of the format grating in the desired storage location.
A first method of data recording involves dispersed nanoparticles absorbing light of a given intensity, transferring the heat from the absorbing nanoparticles to a thermal-acid generator, initiating the generation of acid and using the thermally generated acid to further polymerize the media. The further polymerization results in recording data by locally altering the previously written format hologram grating.
A second method of data recording involves use of a photosensitizer absorbing light of a given intensity and wavelength, transferring an electron from the photosensitizer to a photo-acid generator, initiating the generation of acid and using the photo-generated acid to further polymerize the media, thereby recording data by locally altering the format hologram grating.
A third method of data recording involves a xe2x80x9ctwo-photon absorbingxe2x80x9d photo-acid generator absorbing light of an increased intensity so as to cause direct excited state two-photon absorption in the photo-acid generator, initiating the generation of acid and using the acid to further polymerize the media thereby recording data by locally altering the format hologram grating
Additionally, the invention also comprises a method for producing high optical quality carbon black dispersions in a polymer matrix using surface functionalization. This method comprises the steps of adding an appropriate mass of carbon black to an acceptable monomer, adding an appropriate mass of a trimethoxy silane derivative to the carbon black-monomer combination, mechanically milling the formulation to lessen aggregation, and filtering the formula to remove aggregated particles.
Preferably, the invention also comprises an optical data storage device comprising the optical data storage medium discussed above and having a format hologram stored within the medium. This optical data storage device may take the form of a disk, a tape, a card or the like.